The present invention relates to a system for reproducing data recorded on an optical disc, and more particularly to a system for reducing noises to be included in a reproduced signal.
FIGS. 8a and 8b show a conventional signal reproducing system. The system comprises a semiconductor laser 10 as a point light source for emitting a laser beam, a half mirror 20 reflecting and transmitting the laser beam, an objective 21 focusing the reflected beam from the half mirror 20 on a recording surface of an optical disc 100. A photodetector 30 comprising photodiodes divided into four quadrants (4D-PD) is provided for detecting the reflected beam from the disc through the half mirror, and an operating circuit 6 applied with signals from the photodetector 30 operates for adding and subtracting the signals and producing a focus error signal and a data signal.
The laser beam emitted form the semiconductor laser 10 is focused on the recording surface of the disc 100 through the half mirror 20 and the objective 21. The diameter of the laser beam is about 0.1 .mu.m at an emitting point.
Since the wavelength .lambda. of the beam and the numerical aperture NA of the objective are determined to .lambda./NA&gt;0.1 .mu.m, a beam spot on the disc 100 is limited to .lambda./NA in accordance with the diffraction limit.
On the disc, information is previously recorded by changing of the optical character of the disc. For an example, a disc having different parts in reflectance will be described. If the coloring matter in a recording film of the disc is previously photosensitized, or by using the difference state of the recording film between crystalline and amorphous states, the information is written on the disc.
The intensity of the reflected light from the disc become high or low by the difference of the reflectance. The light is focused on the photodetector 30 through the objective 21 and the half mirror 20. When passing through the half mirror, the reflected light is astigmatized, causing the cross sectional shape of the reflected light to be elliptic. The photodetector 30 is disposed at the spot between the elliptic portions. The beam spot is detected by the photodetector having a detecting surface larger than the diameter of the beam spot so that the beam spot is detected as one signal.
FIG. 8c shows respective images of the beam spots on the photodetector.
However, the detected signal may include noises. The noises include first noises dependent on the laser beam and external noises which do not have relation to the scanning speed for the bits, and further include second noises dependent on the shape of the pit which has relation to the scanning speed. In the conventional system, it is impossible to separate the first noises from the second noises. In order to improve the SN ratio, it is necessary to use a semiconductor laser emitting the beam of a large power. However, such a laser causes reliability of a recording medium to reduce and the price of the reproducing device to increase.
Japanese Patent Publication No. 61-8494 discloses an optical reproducing system. In the system, a part of a beam applied to a disc is covered to an electric energy, and the variation of the intensity of the light is detected so as to control the laser, thereby stabilizing the laser power.
In such a system, it is necessary to provide a high bandwidth modulator and a beam splitter for detecting the variation of the light, which causes the structure of the system to complicate.